This invention relates to a transmitter-receiver circuit for use in a half-duplex communication system or in a system arranged for an alternate, one way at a time, independent transmission.
Generally, in a communication system, transmitter and receiver are separately provided for signal transmission and reception and, in aerial communication, the use of separate antennas are often used for transmission and reception. Such a communication system is characterized by its large size in structure and is undesirable for use as a simple and compact communication means. Particularly, where antennas are employed separately for transmission and reception, leakage radiation of the local oscillation occurring from the receiving antenna may interfere with the signal sent from the transmitting antenna and adversely affect the effective directivity of the latter and the modulation index. In some systems, transmitter and receiver separately provided are arranged to share a single antenna, but such a system necessitates an additional component, such as a duplexer, a nonreciprocal circuit or a switchover unit, for separation between transmitting and receiving signals and actually, due to more or less the imperfection of such additional components, some interference between the signals is unavoidable and not negligible. For an example of the use of a duplexer for the signal separation, reference may be made to an article by William L. Firestone entitled "Multiplexing Klystrons" in Electronics for Communication Engineers, pages 320-325, published by McGraw-Hill Book Co., in 1952 (particularly see FIG. 6 on page 323). Reference may also be made to an article by A. Egger entitled "FM 120/5000 Radio Link System" published in IEEE Transactions on Communication Technology, Vol. COM-14, No. 4, August, 1966, page 449, 455 (particularly FIG. 7), for an example of signal separation employing a nonreciprocal circuit.